I have sketchup skp files available in thumbnail preview in xnviewmp, I use xnviewmp to categories it because its faster when you need to find it again. It would be great if xnviewmp can support more 3d assets thumbnail even just the preview thumbnail would help a lot, 3d assets examples are 3ds, obj, fbx, skp, dwg.

...and STL. Don't forget STL. It is de facto standard in 3D Printing industry. It has much bigger exposure than formats you mentioned, except dwg which is, in fact, a 2D format and it is (half?)supported by XnView MP (it extracts the embedded thumbnail but does not view the entire file)

The STL related request is here with the necessary code to implement it.